1. Technical Field
The invention relates generally to an improved vehicle suspension system. More particularly, the invention relates to air spring suspension systems for land vehicles which include a parallelogram kinematic movement. Specifically, the invention relates to a parallelogram suspension system which is roll stable and resistant to lateral deflection.
2. Background Information
Suspensions are available in the prior art which utilize air springs to provide a comfortable ride, even for large over-the-road trucks and other heavy vehicles. The air springs are typically used in industrial vehicle single axle units tandem or multi-axle arrangements of two or more axles which are either driven or non-driven.
One drawback of air spring suspensions is that an air spring is essentially an air inflated bag and requires auxiliary stabilization. An air suspended axle must have separate independent mechanical location and attitude controls and stabilized components or it will not function. Absent stabilization, the air spring will extend to its maximum length or width in the direction of least resistance. Also, uneven transverse load distribution on a vehicle supported on unstable air springs will cause vehicle lean and tip-over.
A significant number of air spring suspensions have been developed which to a greater or lessor extent, control axle location and attitude. A number of suspensions that have been developed are roll rigid, while others are roll flexible, each generally being designed for a specific application. The most common roll rigid configuration is the trailing beam type suspension, most of which use the axle as a torsion rod to provide roll rigidity.
Another type of suspension which has been developed is the parallelogram suspension which is not inherently roll rigid, and does not inherently provide lateral stiffness. Again, ancillary devices such an anti-roll bars, track bars or guide mechanisms have been utilized to stabilize typical parallelogram designs. As such, parallelogram type suspensions, even with the ancillary devices attached, were often only suitable for low center of gravity loads, or on specialized vehicles stabilized by other vehicle suspension mechanisms.
Trailing arm suspensions are brake reactive. That is, when the vehicle brakes are applied, the suspension will tend to compress thereby reducing the suspensions effectiveness. Similarly, when the brakes are applied as the vehicle moves in reverse, the suspension will tend to raise up, and pivot about the single trailing arm pivot, again reducing the suspensions effectiveness. Further, most trailing arm suspensions suffer from dock walk such that they move toward or away from the loading dock as the suspension moves up or down with the brakes locked. This movement is caused from air draining off the air springs, or as a result of loads added to or removed from the vehicle, or the temperature changes that occur as the trailer remains parked by the dock. Dock walk occurs, primarily because of rotation of the beam, axle and tire assembly when the brakes are locked. As the suspension travels vertically with the brakes locked it rotates the tires causing the tires to move the vehicle horizontally. If the trailer is positioned adjacent a dock, it causes the trailer to move toward or away from the dock as a result of the movement or rotation about the single pivot point.
Similarly, trailing arm suspensions do not utilize the air springs full capacity as the air spring plates are not parallel in extreme operating positions, again as a result of the trailing arm pivoting about a single pivot point.
Parallelogram suspensions were developed to solve a number of the problems associated with trailing arm type suspensions. However, parallelogram suspensions create problems not present in trailing arm type suspensions. Specifically, parallelogram suspensions are not inherently roll rigid or provide lateral stiffness. Parallelogram suspensions have been found to be a significant advancement over the prior art as they provide a relatively stable, safe, and comfortable ride for all types of loads. Some of these parallelogram suspensions are included in U.S. Pat. Nos. 4,114,923, 4,132,432 and 4,309,045.
Advantages of the parallelogram type air spring suspensions include that the air suspended axle in a parallelogram suspension moves a very short linear distance between the loaded and unloaded positions and has no rotational component to the motion. This reduces the problem of dock walk inherent in trailing arm type suspensions.
Further, the parallelogram stabilized suspension permits the air spring's full-load capacity to be utilized. The top and bottom air spring plates remain substantially parallel throughout the full range of air spring travel whether the vehicle is fully loaded or unloaded. Specifically, when the air spring is mounted on the parallel moving link of the parallelogram it allows the utilization of the air springs full travel and full load capacity. In comparison, in the typical trailing arm design where the air spring travels in an arc and "fans" open stretching the rearmost fibers of the spring while not utilizing even the full travel of the forward part of the air spring.
Yet a further advantage of the parallelogram suspension is its inherent ability to maintain a constant caster angle for steerable or caster steering axles which are often utilized in auxiliary axle suspensions for tractors and trailers.
Parallelogram type suspensions are also typically not brake free active. That is, they do not dive or raise when brake torque is applied to the suspension system.
The parallelogram suspension inherently provides the above advantages, and also locates the axle relative to the longitudinal axis of the vehicle by controlling the forward and rearward motion of the axle relative to the frame. Moreover, a parallelogram suspension also controls the path which the air spring follows as it operates to take up irregularities in the road surface. However, the parallelogram suspension alone does not stabilize the air spring. Specifically, the parallelogram itself does not provide lateral stability to the suspension system.
Lateral forces act on a suspension system in a variety of ways with the most common being that lateral forces act on a suspension as the vehicle negotiates a turn. As the vehicle turns, shear stresses act between the tire and the road surface causing a lateral stress to be transferred through the tire-wheel assembly to the axle. The axle, being rigidly attached to the suspension, transfers the lateral forces into the parallelogram causing it to laterally deflect. This lateral deflection can be extreme and substantially limits the usage of parallelogram suspensions. Lateral force may be strong enough under certain loading conditions that the tires contact the vehicle frame rails.
It is thus necessary to provide mechanical means for controlling lateral forces on the suspension and its various members. One typical non-parallelogram type suspension where lateral forces are mechanically controlled is shown in U.S. Pat. No. 3,140,880 to Maser in which air springs are disposed between two vertically swinging control arms to which the axle is also attached. One feature of this suspension is that much of the lateral force is controlled by a strong, relatively rigid attachment between the axle and the control arms. As such, the lateral force is taken up by the attachment between the control arm and the axle. While the suspension system of this patent presumably functioned for the purpose for which it was intended, it suffered from dock walk, brake reactivity, and did not utilize the full load carrying capacity of the air spring. Moreover, it is desirable to provide for greater flexibility between the axle and the control arms, while still maintaining sufficient lateral stability and thus increase the suspensions roll stability. Thus, the second problem inherent in parallelogram air spring suspensions is that they are not roll stable.
Roll instability refers to the counteracting forces operating on the ends of an axle causing one end of the axle to raise relative to the frame a distance greater than the other end of the axle. Roll instability is encountered when the vehicle frame tilts or rolls relative to the axle; for example, when the vehicle negotiates a turn such that the centrifugal and acceleration forces reduce the downward forces acting on the inside wheel of the turn, and increase the downward force acting on the outside wheel of the turn. Roll instability is also realized when the axle moves relative to the frame; for example, during diagonal axle walk.
Diagonal axle walk occurs when the axle moves relative to the vehicle frame which occurs when the wheels at the opposite ends of the axle encounter unlike irregularities in a road or off-the-road surface, such as when one wheel rides over a curb. As the wheel rides over the curb or bump, an upward force acts on that wheel, and a counteracting downward force acts on the wheel not riding over the curb. If the suspension is unable to provide flexibility between the axle and the frame as the tire-wheel assembly travels over the curb or ground irregularity, or alternatively to provide flexibility between the axle and the frame as the vehicle negotiates a turn, the suspension will be too roll rigid, and may cause axle breakage or over stress vehicle components such as the frame. Roll rigid suspensions are used to stabilize high center of gravity vehicles like highway trailers, and are most critical in applications such as tank or dump trailers and vans having high volume boxes and thus a relatively high center of gravity. In these applications, only enough roll compliance is permitted to allow the axle suspension combination to negotiate uneven terrain without unduly stressing the vehicle frame or axle. Typically, the roll angles of axle to frame are limited to 2 to 3 degrees in roll rigid suspensions. That is, if all the load were transferred to the tire or tires on one side of the vehicle and the tire or tires on the other side of the vehicle are completely off the ground, the angle of the axle relative to the frame reaches only about 2 to 3 degrees for a typical roll rigid suspension.
Conversely, roll flexible suspensions are used on low height vehicles and multi-axle vehicles which are stabilized by only some of the suspensions and only want to increase the load carrying capacity of the unit with the addition of a flexible suspension. In these applications where tractive effort is paramount, the suspension must be flexible to allow the tires to remain in contact with the ground. Specifically, if a suspension is added to provide an increased total vehicle weight, it is often beneficial for the suspension to be relatively flexible. This flexibility ensures that the tires remain in ground contact to assure that the increased carrying capacity of the axle is evenly transmitted through the frame to the ground, without inducing undue stress in the vehicle frame structure. Regardless of whether a roll rigid or roll flexible suspension is utilized, the suspension must be roll stable and provide the proper roll and lateral control needed to assure that the total vehicle is stable.
Attempts have been made to provide additional resistance to lateral forces while simultaneously allowing the frame to "roll" in a controlled manner relative to the axle without regard to the vertical loads supported by the air springs. Prior attempts to provide additional roll resistance include the addition of stabilizer bars, roll bars or torsion bars secured between the suspension and the frame, or by stiffening the connection between the axle and the control arm as described above in typical trailing beam suspensions. One such suspension is shown in U.S. Pat. No. 5,083,812.
Such improvements, however, may nevertheless affect the handling and ride of the vehicle, and transfer the load caused by the lateral or roll control forces to the frame thereby overstressing vehicle components. Such systems are frequently more complex, having many moving components, and may also have limited application, especially where the vehicle center of gravity is over a predetermined height.
Therefore, a need exists for an air spring suspension which is parallelogram stabilized and is roll stable, but which is also resistant to deflection from lateral forces.